Owing to their simple construction and simple mode of operation, rotary piston engines theoretically may be used for a wide range of applications. The constructions used so far have been employed mainly for such hydraulic-technical applications where oil or other self-lubricating and non abrasive fluids are transported and the fluids are free of particles. The reason for this restricted use is that rotary piston engines, as presently designed, possess properties that discourage conveyance of abrasive materials through the engines since if they were, the wear would become too considerable, affecting engine reliability and performance.
The weakest point of these prior-art rotary piston engines is found in the movable partition wall that separates the suction and pressure sides (in pump applications) from one another. This partition wall usually is designed as a movable slide means that abuts against the rotary piston surface and by means of a force applied thereon is pressed in the direction towards the centre axis of the piston to provide a sealing action.
The pressure is applied by spring means, hydraulic means or in any other manner. The pressure requirements of the slide means depend on the pressure that has built up inside the engine and on the speed of rotation as well as on the configuration of the sealing surface (curve line) of the rotary piston.
In these rotary piston engines the slide means is carefully mounted in a groove/seat and moves with narrow tolerances in outwards and an inwards direction motion. The construction is sensitive to wear and other mechanical influences.
Another way of designing the partition wall is described in U.S. Pat. No. 1,172,505 wherein an oscillating yoke having two surfaces of contact that abut against a rotary piston forms the partition wall between the pressure and suction sides in a rotary piston engine that operates as a pump. The oscillating yoke and the rotary piston is configured to ensure that irrespective of the angle assumed by the rotor the yoke will abut against the rotor by means of two surface of contact. In addition, the yoke is mounted on a shaft that is attached to the pump housing in a fixed point of attachment about which point the yoke pivots.
A necessary prerequisite for this construction is that the sum of the distance from the centre of the rotary piston to one of the yoke tips and the distance from the centre of the rotary piston to the other yoke tip remains constant at all times. In practice, this means that the sealing force provided by the sealing yoke is directed diametrically against the rotor. A drawback of this design is the restriction that the above-mentioned condition imposes on the geometry of the rotary piston. Another disadvantage is that the sealing yoke is mounted on a shaft or similar means, which adds to the number of components that are exposed to wear and contribute to wedging of particles. A third disadvantage is that the yoke has large surface areas that are exposed to the medium on the pressure as well as one the suction sides, for which reason the output pressure acting on the rotary piston at the surfaces of abutment of the yoke increases rapidly at the counter-pressure rises and the negative pressure increases. Negative pressure on the suction side and excess pressure on the pressure side cooperate to increasing the pressure of abutment of the yoke on the pressure side. In addition, another consequence of this design is that a large proportion of the pump housing volume becomes inactive and does not take part in the pressure build-up by the rotary piston, in the fluid sealing, and so on.
Patent Specification U.S. Pat. No. 4,047,857 describes another constructional solution for sealing the rotary piston engine. This construction comprises at least one flexible curved membrane which is secured in a rotor rotating about a stator located interiorly thereof. The sealing membrane consists of a cylindrical stationary bearing portion which is fitted in the rotor to which the membrane is secured and around which the membrane oscillates in operation. The curved membrane abuts against the surface of the stator and is adapted to pivot into a curved recess in the rotor.
Quite apart from the technically complex manufacturing method, this construction, like the previous one, is formed with large pressure surfaces with resulting high pressure of abutment upon rising counter-pressures on the outlet side, and the negative pressure on the suction side co-operates with the excess pressure on the pressure side to further increase the pressure of abutment of the membrane against the surface of the stator during the rotation. In addition, the structure is extremely sensitive to the presence of particles that may easily wedge themselves in the gap formed between the upper face of the membrane and the recess in the rotor.
A feature common to all prior-art rotary piston engines operating on the principle of employing a sealing means in the shape of a yoke or a resilient membrane is that the yoke or the membrane pivots about a fixed centre line at a predetermined distance from the axis of rotation of the rotary piston or the rotary body.